An electron microscope has a beam generating column in which a beam of electrons is generated, shaped and focused by means of controlled electrostatic and/or magnetic fields. This beam is then scanned across a sample to be analysed.
The electron microscope has a number of user controllable operating parameters that define the operating conditions of the microscope. Those parameters include the accelerating (EHT) voltage applied to an accelerating electrons in the column. The voltage thus being related to the kinetic energy of electrons in the beam. Certain types of microscope also have a number of apertures (of different sizes) in a diaphragm in the column and elements for directing the beam through a selected one of those apertures. This provides control over the probe current constituted by the beam. Generally, for example, high probe currents (and hence large apertures) are used for analysing xrays emitted from a sample in response to the probe current, whilst lower currents are required for high resolution work, such as secondary electron imaging.
In addition, electron microscopes can have a high current mode in which the probe current is boosted by means of a condenser lens in the column and a variable pressure mode in which the sample is maintained in a sample chamber in a gaseous atmosphere above a given threshold pressure, for example, 1 Pa. These other modes also constitute user controllable operating parameters for defining the operating conditions of the microscope.
When operating an electron microscope, it is often necessary to adjust certain further operating parameters, in particular the deflection and/or focusing elements in the electron column, to ensure correct electron optical alignment of the column under the selected operating conditions. This results in time consuming adjustments every time the operating conditions are altered.
In some cases, the column is set-up precisely under a few well defined sets of operating conditions, and the various associated further operating parameters of the column are stored as factory settings. These values can then be accessed in a table, possibly using interpolation to obtain values for the further operating parameters where the conditions do not accord to any factory setting. However, the calibration is usually fixed and cannot be adjusted by the user, so that if the electron optical properties of the instrument change over time, the instrument requires re-calibration. Furthermore, it is only possible to store the settings for a few selected sets of conditions, thus permitting significant errors when the operating conditions fall between calibrated values of individual parameters.
Some microscope allow the user to store a set of operating conditions, and associated further parameters under a specified name. These conditions and parameters can be restored to re-create a prior operating regime. Unfortunately, this approach also has several problems. Firstly, when a large number of different conditions are stored, it becomes difficult to remember the contents of each data set. Furthermore, the user has little control over the data stored in each set.